Feedback control apparatus



June 21, 1966 D, BURK D 3,257,510

FEEDBACK CONTROL AP PARATUS Filed Oct 15, 1962 3 Sheets-Sheet 2 RELATWERESPONSE \N DE x FREQUENCY \N CYCLES PER SECOND INPUT INVENTOR. MAHLOND. BURKHARD BY m, 2%; mu

June 21, 1966 M. D. BURKHARD 3,257,510

FEEDBACK CONTROL APPARATUS Filed Oct 15, 1962 5 Sheets-Sheet &

PHASE. 5 $HH=T (b i1,

PHASE $H\FT +\2o PHASE SH\\=T +240 Jig. 8

INVENTOR. MAHLON D. BURKHARD HELL 5..

3,257,510 rnuunaorr CONTROL ArPAuArUs Mahlon ll). Eurkhard, Hinsdale,Ill., assignor to Industrial Research Products, lino, Franklin Park,ill, a corporation of Delaware Filed Oct. 15, 1962, Ser. No. 230,420 5Claims. (Cl. 179-1) This invention relates to a new and improvedfeedback control apparatus. More particularly, the invention relates toa-feedback control apparatus that is effective to prevent oscillation ina system comprising an output device that is coupled back to its inputdevice through a feedback medium producing irregular gain at differentfrequencies. The invention is particularly advantageous as applied to apublic address or similar sound system, and is described in thatconnection, but may also be applied to other applications presentingsimilar oscillation problems.

System oscillation, resulting in howling or squeak ing, is a familiarphenomenon in the operation of public address and similar sound systems.The oscillation results from acoustical feedback from the loudspeaker ofthe system to the microphone. Some of the feedback can be minimized byemploying electrical filters to minimize response peaks in the system,particularly in the speakers. Direct acoustical feedback from thespeakers to the microphone can also be reduced by proper placement ofthese devices and, on occasion, by the use of directional microphonesand speakers. These expedients, however, are not-eifective with respectto acoustical energy that is reflected from the walls of the room, whenthe system is located indoors, or from other encompassing structures.

The reflected sound, sometimes known as reverberant sound, is not ofconstant amplitude at all frequencies within the range of operation ofthe system. Rather, there are marked variations in the amplitude of thesound impinging upon the microphone, depending upon frequency. Thedistribution of the acoustical feedback peak frequencies over thefrequency range of the system may vary substantially, depending upon thelocation of the microphone and the speakers within a given structure.Thus, although individual peak frequencies might be reduced by fixedfilters, effective results could not be achieved if the microphone weremoved during the course of operation of the system, as frequentlyoccurs. Furthermore, the peak frequencies are so numerous as to makeindividual filtering an economically prohibitive procedure.

Reverberant-sound oscillation can be avoided by incorporating afrequency shifting circuit between the microphone and the loudspeaker ofthe public address system to shift all frequency components of the'signal supplied to the speaker by a constant amount. Optimum operationof a system of this kind is achieved with a frequency shift generallycorresponding to the spacing between the peak response frequencies ofthe location in which the system is employed. The-actual amount offrequency shift is not extremely critical. On the other hand, thefrequency shift utilized in systems of this kind cannot be at too high afrequency without appreciably distorting the output of the system.

Frequency shifting apparatus as heretofore proposed for the correctionof feedback difficulties in public address and similar systems has beenrelatively complex and expensive. In one proposed system, the initialsignal,

after amplification, is first modulated with a high frequency carriersignal from a crystal controlled oscillator. The signal is subsequentlydemodulated with a carrier signal from a second crystal controlledoscillator, the two.

carrier signals being very close in frequency. A system of this kind isdependent upon the maintenance of a constant frequency differentialbetween the two frequency oscillators and requires relatively complexand expensive operating circuits.

It is a principal object of the present invention, therefore, to providea new and improved feedback control apparatus that is effective tominimize instability and oscillation in a public address or similarsystem of the kind entailing substantial feedback, with differentamplitudes at varying frequencies, over a given frequency band. Arelated object of the invention is to overcome the difliculties anddisadvantages of previously known feedback control apparatus.

A more specific object of the present invention is to provide acontinuously varying phase shift affording an effective frequency shiftbetween the input and output devices of a public address system or thelike, minimizing the tendency of the system to oscillate, withoutrequiring the use of critical frequency elements such as crystalcontrolled oscillators and modulators.

Another object of the invention is to afford a means for quickly andconveniently varying the rate of phase shift in a continuousphase-shift-feedback control apparatus for a public address or similarsystem entailing feedback at varying amplitudes for differentfrequencies over a relatively broad frequency band, thereby making thefeedback control apparatus of the invention readily adaptable to affordoptimum performance in different environments.

A specific object of the invention is to provide an inexpensive andsimple mechanical system for controlling frequency shift in a feedbackcontrol apparatus for a public address or similar system.

Thus, the present invention is directed to feedback control apparatussuitable for use in a public address or like system that includes inputmeans for developing an initial signal, a signal channel comprising anamplifier and ut-ilization means, such as a loudspeaker, for utilizingthe amplified signal. In particular, the feedback control apparatus isapplicable to a system of the kind in which the utilization means iscoupled back to the input means through a feedback medium producingvarying gain at different frequencies within a given frequency band.Feedback control apparatus constructed in accordance with the inventioncomprises a rotary resolver interposed in the signal channel between theinput and utilization means of the system; the rotary resolver includesat least two input stages and an output stage with the out put stagebeing rotatable relative to the input stages to vary the interstagecoupling of the resolver. The control apparatus further includes phaseshifting means for developing two intermediate signals eachcorresponding to the initial signal developed by the input means of thesystem but shifted in phase, relative to each other, by a predeterminedamount. In addition, the apparatus includes drive means for rotating theresolver at a predetermined frequency, preferably below the frequencyrange over which the system operates. As a result of rotation, theoutput stage of the resolver produces a utilization signal thatcorresponds to the initial signal from the input means of the system butis of continuously varying phase, and hence shifted in frequency,relative thereto. Switch- 3,257,510- Patented June 21, 1966 ing means orother appropriate means are provided for reversing the effectivedirection of rotation of the resolver to change the sign of thefrequency shift.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings which, by way of illustration, show preferredembodiments of the present invention and the principles thereof and whatis now considered to be the best mode contemplated for applying theseprinciples. Other embodiments of the invention embodying the same orequivalent principles may be made as desired by those skilled in the artwithout departing from the present invention and the purview of theappended claims.

In the drawings:

FIG. 1 is a block diagram, partly schematic, of feedback controlapparatus constructed in accordance with a preferred embodiment ofthepresent invention, shown in connection with a conventional publicaddress system;

FIG. 2 illustrates a typical variation in sound pressure amplitude,relative to frequency, at a given location in a room;

FIG. 3 is a chart illustrating the additional gain that can be employedsafely in a public address system in which the present invention isincorporated;

FIG. 4 illustrates the overall frequency response to the feedbackcontrol apparatus illustrated in FIG. 1;

FIG. 5 is a detailed schematic diagram of a phase splitting circuit usedin the feedback control apparatus of FIG. 1;

FIG. 6 illustrates an alternate form of rotary resolver for the feedbackcontrol apparatus of FIG. 1;

FIG. 7 illustrates a feedback control apparatus comprising anotherembodiment of the present invention; and

FIG. 8 illustrates yet another embodiment of the invention.'

FIG. 1 illustrates a public address system comprising a microphone orother suitable input device 10 connected to a conventional audioamplifier 11. Amplifier 11 is provided with an output circuit that iscoupled through a feedback control apparatus 12, constructed inaccordance with the present invention, to a suitable power amplifier 13.The power amplifier 13, in turn, is connected to utilization means whichin this instance comprises one or more loudspeakers 114. It is thus seenthat the system illustrated in FIG. 1 is conventional in form except forthe incorporation of the feedback control apparatus 1 2 in the signalchannel coupling the input device 10 of the system to the output device14.

As shown in FIG. 1, some of the output from speaker 14 may beacoustically coupled directly back to microphone 10 along a path 15.Direct acoustical feedback coupling of this kind can be minimized byproper placement and orientation of the speaker and the microphonerelative to each other. Additional feedback is produced, however, byreflection of the sound from speaker 14 to microphone 10' along avariety of different paths which may be of varying length. This aspectof the system is generally illustrated by the reflection feedback paths16 and 17. In an actual system, of course, the total number of differentacoustical feedback paths between speaker 14 and microphone 10 is verylarge in number and the lengths of these paths vary so substantiallythat there are marked differences in the gain, or amplitude, ofdifferent frequency components of the reverberant sound impinging uponmicrophone 10.

FIG. 2 is a graphic representation of the relative acoustical responseor sound pressure level in a typical room, over a range of frequenciesin the neighborhood of l kilocycle. The average value of the soundpressure level is taken as the zero point on the response scale in FIG.2; superimposed upon this average level is the contribution of the soundreflected from the walls of the room or other objects in the room, asrep resented by the curve 21. As can be seen from FIG.

2, at some frequencies the reverberant sound provides increased soundpressure (for example peaks 22, 23, 24). At other frequencies, thereverberant sound reduces the sound pressure level, at the samelocation, as evidenced by the valleys 25, 26, 27 and others in curve 21.

Feedback control apparatus 12 (FIG. 1) takes advantage of thisirregularity in frequency response by reducing the sound pressure peaksto lower values, thereby decreasing the tendency for the public addresssystem to oscillate. The lower limiting value, of course, is the averagesound level in the room, the zero level of the response scale in FIG. 2.At the same time, feedback control apparatus 12 effectively raises theminimal points or valleys in the room frequency response characteristictoward the average sound pressure.

Feedback oscillation, in a public address system or the like, occurs atfrequencies for which the phase of the feedback signal is such as to aidin synchronism with other signals entering the microphone. If thissynchronism is disrupted by phase shifting or frequency shifting, thetendency toward system oscillation is minimized. A finite time isrequired for the build-up of oscillation. If the phase of the feedbacksignal is changed during a period less than the buildup time requiredfor oscillation, then it is necessary for build-up toward oscillation tobegin anew. Consequently, if the phase of the signal is changedcontinuously, producing an effective frequency shift, there is aneffective increase in the sound level required to initiate oscillationin the system. Control apparatus 12 effectively interrupts the phasecoherence of the acoustically fed back signal from speaker 14 tomicrophone 10, making it possible to increase the overall gain of thesystem and thereby permitting a substantial increase in the effectivesignal sound level in the room.

Although the foregoing discussion is based upon considerationspertaining to the use of a public address system in an enclosed spacesuch as a room, control apparatus 12 is equally effective in any similarapplication in which acoustical or mechanical feedback is present and inwhich the frequency response over the feedback path is of irregularnature.

As shown in FIG. 1, feedback control apparatus 12 comprises phaseshifting means including a first phase delay circuit 31 and a secondphase delay circuit 32, both of which are coupled to the output ofamplifier 11. The phase shifting means represented by circuits 31 and 32may constitute any of a number of conventional phase splitting devices.In a convenient and preferred arrangement, circuit 31 comprises a firstseries of resistance-capacitance phase shift elements and circuit 32constitutes a similar series of RC phase shift elements. A specificcircuit of this kind is described hereinafter in connection with FIG. 5.Each series of phase shift circuits provides a continuous andapproximately constant change of phase over a relatively broad frequencyrange determined by the number of stages in the series. In the presentinstance, the two phase delay circuits are adjusted to afford differentphase shift characteristics, the phase difference at the outputs beingestablished at approximately It is necessary that the phase differencebetween the output signals of circuits 31 and 32 be approximatelyconstant over a frequency range corresponding to at least a substantialportion of the operating frequency range of the public address system.The preferred circuit illustrated in FIG. 5 may be replaced by any phaseshifting apparatus effective to achieve this end.

The output of phase delay circuit 31 is connected to a movable contact33 in the first section of a double-pole double-throw switch 34.Similarly, the output terminal of circuit 32 is connected to a secondmovable contact 36 of the switch. The first movable contact 33 of switch34 is engageable with fixed contacts 37 and 39; the other movablecontact 36 engages either of two fixed contacts 38 and 40.

Feedback control apparatus 12 further includes a rotary resolver 41which in this instance is a rotary transformer. Resolver 41 includes a,pair of input stages comprising primary windings 43 and 44, and anoutput stage comprising a secondary winding 45. Input windings 43 and 44are disposed in quadrature relation to each other and terminate at acommon terminal 46 which is grounded. The secondary winding 45 of theresolver is rotatable relative to the two primary windings 43 and 44,rotation of winding 45 being effective to vary the coupling between theoutput winding and each of the two input windings. It is the outputwinding 45 of the resolver that is connected to power amplifier 13 ofthe public address system. i

The output stage of rotary resolver 41, winding 45, is mechanicallyconnected to a resolver drive means 47. Drive means 47 may comprise aconventional electrical motor and may be a constant speed device. On theother hand, it is frequently desirable to provide some means for varyingthe speed of rotation of resolver winding 45; to this end, it may bedesirable to utilize a variable speed electrical motor or to provide avariable drive ratio device 48 as a part of the drive means 47 forrotatable winding 45.

In considering operation of feedback control apparatus 12, it is seenthat the signal E from amplifier 11 is applied to both of the phasedelay circuits 31 and 32. As noted above, the output signals from thephase delay circuits are displaced by a phase angle of 90 relative toeach other. Accordingly, the output signal E from circuit 31 may berepresented by the expression E :E sin wt sented by the expression (2) EzE cos wt=E sin (wt+90) The first intermediate signal E is supplied toinput winding 43 of resolver 41 and the other intermediate signal E isapplied to the second input winding 44 of the resolver. Since thesecondary winding 45 of the resolver is inductively coupled to both ofprimary windings 43 and 44, the signal E developed in the secondary oroutput winding of the resolver is a function of both of the inputsignals E and E Thus, the output or utilization signal E from theresolver that is applied to power amplifier 13 may be represented by theequation (3) E =E sin 6+E cos 0:E sin (wt-Hi) in which 0 is theinstantaneous angle of resolver secondary 45 relative to the referenceaxis of the primary windings 43 and 44 of the resolver.

The output winding 45 of resolver 41, however, is rotated continuouslyby resolver drive 47, and the rate of phase change in the output signalE, from the resolver is dfl/dt. Accordingly, the utilization signal Emay also be represented by the expression (4) E =E sin w+ )t From FIG.2, it can be seen that the spacing between response peaks for thelocation in which the public address system is used is of the order offive to ten cycles per second. Consequently, a frequency shift of theorder of five cycles per second should be effective to break up theotherwise potentially coherent feedback pattern of the system andeffectively minimize the tendency of the system to oscillate at peakssuch as the peak 23.

To accomplish this, with resolver 41, drive means 47 is adjusted toafford a rate of rotation for resolver secondary 45 of five revolutionsper second. Each revolution of secondary 45 accomplishes a complete 360phase shift and thus constitutes a frequency change of one cycle persecond. Accordingly, if drive means 47 rotates secondary winding 45 at300 r.p.m., five revolutions persecond, the resulting rotation of thesecondary produces a total frequency shift in signal E as compared withthe-initial input signal E, of five cycles per second. A frequency shiftof six cycles per second can be accomplished by operating drive 47 at aspeed of 360 r.p.m., and corresponding changes in the drive speed may beaccomplished to afford other frequency shift values, depending upon thespacing between the peaks and valleys in the response curve, FIG. 2.

From the foregoing description, it can be seen that feedback controlapparatus 12 may be considered in terms of either a frequency shift-ingdevice or a phase changing device. The apparatus afiords a frequencyshift of a given value, as for example, five cycles per second, eachtime a signal passes through it. Thus, an acoustical signal applied tomicrophone '10 is shifted five cycles per second in frequency before itis emitted into the room by loudspeaker 14. The sound fed backacoustically to microphone 110, along paths i15-17, is again shifted infrequency by the same amount as it is passed through the signal channelof the public address system and back to speaker 14.

As the signal returns repeatedly for reamplification by the publicaddress system, it is shifted in frequency by a total of Sn cycles persecond, where n is the number of times the signal passes through thesystem. As a consequence, the loop gain of the system is maintained atless than unity as long as the sound being fed back at relatively highlevels, corresponding to the peaks of the response curve (FIG. 2), isshifted into regions of the re sponse curve having low response levels.If the sound level in the room were uniform and independent offrequency, the frequency-shifted signal would still be fed back at alevel that could lead to oscillation, but a room construction affordingsuch characteristics is virtually unknown.

An inductively coupled resolver, such as resolver 41, providessubstantial latitude in selection of turns ratios and other performancecharacteristics. Consequently, a wide choice of design parameters,particularly with respect to the output impedance of the resolver, isavailable. On the other hand, capacitive or resistive resolvers can beem ployed, a capacitive device being discussed hereinafter in connectionwith FIG. 6.

One advantageous feature of the feedback control apparatus 12 is theease and convenience of changing the amount of frequency shift (the rateof phase change) by the simple expedient of varying the speed ofresolver drive 47. As noted above, the rate of rotation can be modifiedby utilizing a variable speed motor as the resolver drive or byemploying a mechanical linkage 48 of variable drive ratio between theresolver drive and resolver secondary 45. This aspect of the inventionenables a simple and expedient adjustment of the operatingcharacteristics of the resolver to meet the conditions of the auditoriumor other environment in which the public address system is used.

Another feature of feedback control apparatus 12 is that it provides forquick and convenient reversal of the direction or sign of the frequencyshift without the addition of circuit elements other than switch 34. Ifswitch 34 is actuated from the illustrated position to the alternateposition, it is seen that the signals E and E are reversed in theirconnections to the primary windings 43 and 44 of resolver 41. Thus, withswitch 34 thrown to its alternate position, signal E is supplied toinput winding 44 and signal E is applied to input winding 43. It can beshown that, under these conditions, the resultant signal E developedacross secondary winding 45 is of the form Thus, the reversal of theinput connections to resolver 41 effected by switch 34 is equivalent toreversing the direction of rotation of secondary winding 45 relative toprimary windings 43 and 44 and reverses the sign or direction of thefrequency shift, a reversal that is difficult and costly with previouslyknown frequency-shift feedback control apparatus.

Of course, the sign of the frequency shift or phase change effected byapparatus 12 can also be reversed by reversing the direction of rotationof resolver drive 47. This can be accomplished by utilizing an electricdrive motor that may be reversed in its direction of rotation, as bychanging the polarity of the input to the motor or by changing theconnections to the field and armature of the motor, in accordance withconventional practice. The feature of reversal of the sign of frequencyshift, whether accomplished by means of a reversible drive or by meansof a switch such as switch 34, is desirable to compensate for variationsamong auditoria in which the apparatus may be employed.

FIG. 3 is a graphical representation of the additional gain available,short of oscillation, in the public address system of FIG. 1 with thefeedback control apparatus '12 incorporated therein, as compared with asimilar system not employing comparable feedback control apparatus. Theadditional gain is plotted as a function 51 of the frequency shifteffected by apparatus 12.

From curve 51 of FIG. 3, it can be seen that, as would be expected, noadditional gain can be realized if the frequency shift afforded byapparatus 12 is equal to zero. Furthermore, very small frequency shiftsof the order of one or two cycles do not have sufiicient effect inbreaking up feedback coherence in the overall public address system andpermit only relatively small increases in total gain of the system. Afrequency shift of as much as four cycles, either advancing orretreating, makes it possible to achieve quite substantial amplitudeincreases without engendering oscillation in the system. Furthermore, aswill be apparent from FIG. 3, there are definite optimum frequenciespermitting maximum gain as indicated by points 52, 53 and 54 in FIG. 3.It should be understood that the curve 1 is applicable only to a givenreverberant room or other specific environment and that this curve canbe substantially different in other environments, thereby establishingdifferent optimum conditions for the amount of frequency shift that willafford the maximum opportunity for advancing the gain of the system.

FIG. 4 is a graphical representation of the relative response offeedback control apparatus 12, plotted as a function 55 of signalfrequency in cycles per second. As can be seen from curve 55, thefrequency response of the feedback control apparatus is virtually flatfrom about 500 cycles per second to over 10,000 cycles per second. Theinitial portion of curve 55 (60 to 500 cycles) shows some low-frequencyloss, due primarily to a somewhat reduced low-frequency response in theparticular phase splitter circuits 31 and 32 employed in the specificapparatus for which the curve was taken. But this slight lowfrequencyloss is not sufficient to cause noticeable distortion in operation ofthe public address system. The

rise in relative response above about 15,000 cycles is due to the factthat resolver 41 exhibits a resonant peak. In this instance the resonantfrequency would be of the order of fifty kilocycles.

FIG. 5 illustrates a specific phase splitter circuit that may beutilized for the phase delay circuits 31 and 32 in frequency controlapparatus 12 of FIG. 1. Circuits 31 and 32, as shown in FIG. 5, eachconstitute an iterated circuit of the general type described in NationalBureau of Standards Report No. 1470 dated February 29, 1952. Phase delaycircuit 31 comprises three individual phase shift circuits 61, 62 and63; circuits 6163 are substantially identical to each other except forcircuit parameters. Thus, the initial phase shift circuit 61 in series31 includes a triode 64- having a control electrode connected to asuitable input stage as by means of an input resistor 65. The cathode oftriode 64 is returned to ground through a load resistor 66 and the anodeis connected to a suitable B+ supply by means of a load resistor 67.

The cathode of the triode is connected through a potentiometer 68 to thecontrol electrode of the next stage 62 in the chain. The capacitor oftriode 64 is connected by a coupling capacitor 69 to the controlelectrode of the succeeding triode.

The next stage of circuit 31 is essentially similar to stage 61. Acoupling capacitor 71 connects the anode of triode in this stage to thecontrol electrode of tube 76 in the next stage. A potentiometer 72connects the cathode of tube 75 to the control electrode of the triode76. Circuit 63 includes a coupling capacitor 73 that couples the anodeof tube 76 to the control electrode of an output triode77. Apotentiometer 74 connects the cathode of tube 76 to the controlelectrode of triode 77.

Phase delay circuit 32 includes 3 stages 81, 82 and 83 paired withstages 61, 62 and 63, respectively, of circuit 31. Stage 31 is similarin construction to circuit 61 and includes a triode 84 having a cathodereturned to ground through a resistor 86 and an anode connected to theB+ supply through a resistor 87. The control electrode of tube 84 isconnected back to resistor 65 to receive the same input signal asapplied to triode 64. The output circuit of stage 81 includes apotentiometer 88 that connects the cathode of tube 84 to the triode forstage 82. A capacitor 89 couples the anode of triode 84 to the controlelectrode in the next stage.

Circuits 82 and 83 are of similar construction. The output circuit ofstage 82 includes a coupling capacitor 91 and a potentiometer 92. Thecorresponding elements in stage 83 are the coupling capacitor 93 and thepotentiometer 94. The latter two impedances are coupled to the controlelectrode of an output triode 97.

Circuits 61 and 81 may be considered as a pair. These two circuits eachproduce an output signal that is shifted in phase as a function offrequency, the maximum phase shift being at a frequency determined bythe circuit parameters selected for elements 68, 69, 88 and 89.Similarly, circuits 62 and 82 function as a pair to afford a furtherphase shift, the frequency of maximum shift being selected to broadenthe overall operating frequency band of the circuit. The third circuitpair 63, 83 represents a continuation of this process, again broadeningthe frequency range. It can be demonstrated that the illustratedcircuits may be constructed to afford a frequency shift of approximatelyin the output signals E and E over a range of to 10,000 cycles persecond.

If desired, a fourth phase splitter circuit pair could be added toincrease the frequency range and to narrow the tolerance of the systemwith respect to deviations from the desired 90 phase shift. On the otherhand, a less expensive system could be constructed with only two sets ofphase splitter circuits. Such a two-stage phase splitter circuit, ifotherwise similar to the circuit of FIG. 5, would ordinarily have anarrower frequency range and would not be able to maintain as closetolerances with respect to deviations from the desired 90 phase shift.By replacing each of the coupling capacitors (e.g. capacitors 69 and 89in the first stage of FIG. 5) with a series RC circuit and replacingeach related coupling resistor (e.g. resistors 68 and 88) with aparallel RC circuit, however, the overall performance of a two-stagecircuit of this general type can be improved to a level comparable toand even better than the three-stage circuit illustrated.

Triodes 77 and 97 serve only as output stages and do not contribute tothe overall phase shift of the system. The illustrated circuit affordsthe necessary 90 phase shift between signals E and E with modification,as noted above, virtually any desired frequency range can be achievedfor the system.

In order to afford a more complete illustration of the presentinvention, circuit parameters for a substantial portion of the circuitillustrated in FIG. 5 are set forth in detail hereinafter. It should beunderstood that these data are furnished merely by way of illustrationand in no sense as a limitation on the invention.

9 Plate load resistors 67, 87, etc. kilohms 12 Cathode load resistors66, 86, etc do 12 Vacuum tubes (all) Type12AT7 Resistor 65 do 500Resistor 68 -Q do 330 Resistor 72 do 390 Resistor 74 do 240 Resistor 88do 180 Resistor 92 do 220 Resistor 94 do 47 Capacitor 69 microfarad0.005 Capacitor 71 do 500.0 Capacitor 73 do 200.0 Capacitor 89 do 0.002Capacitor 91 do 500.0 Capacitor 93 do 2000 13+, volts D.C. +150 Theembodiment of FIG. 1, utilizing the specific circuit shown in FIG. 5,affords substantial improvement in the operation of the public addresssystem as evidenced by the increased permissible amplification, FIG. 3,and the essentially fiat response, FIG. 4. There is little or noevidence of modulation distortion in the audio output of the system. Thesystem does not require the use of crystal controlled oscillators orother critical frequency determining elements; nevertheless, it affordssubstantially greater versatility than previously known systems withrespect to changing the frequency shift to suit individual room conditions.

FIG. 6 illustrates a simple rotary resolver 101 that may be incorporatedin feedback control apparatus 12 (FIG. 1) in place of the resolver 41illustrated therein. Resolver 101 includes four capacitor plates 102,103, 104 and 105 each shaped to afford a quadrant section of a compositecylindrical structure. Plates 102 and 104 are electrically connected tothe opposite ends of a center-tapped secondary winding 122 of an inputtransformer 123, the tap on winding 122 being grounded. Similarly,plates 103 and 105 are connected to the opposite ends of a centertappedsecondary winding 124 of a second input transformer 125. 1

Resolver 101 further includes a rotor 106 upon which a single capacitorplate 107 is mounted. Plate 107 covers approximately 90 of the surfaceof rotor 106' and is electrically connected to a slip ring 108 to afforda means for coupling an output circuit to the plate. In FIG. 6, therotor 106 is shown displaced from the stationary capacitor plates102-105; in actual use, the rotor would be located within thecylindrical configuration afforded by the stationary capacitor plates.

In operation, capacitor plates 102 and 104 afford the first input stagefor the rotary resolver 101 and capacitor plates 103 and 105 constitutethe second input stage for this resolver structure. Signal E is appliedto plates 102 and 104 in push-pull relation, through transformer 122,and signal E is applied in phase opposition to plates 103 and 105,through transformer 124. Rotor 106 is rnechanically connected toresolver drive 47 (FIG. 1) and is rotated at a rate commensurate withthe desired frequency shift to be imparted to the utilization signal Ederived from the slip ring 108 upon the rotor.

Operation of rotary resolver 101 is essentially similar to the resolverdescribed in connection with FIG. 1 except that output plate 107 iscapacitively coupled to the input stages rather than being inductivelycoupled as in resolver 41. Again, to achieve a five cycle per secondfrequency shift with resolver 101, the rotor must be rotated at a speedof 300 revolutions per minute, since each complete cycle of rotor 106 isequivalent to a total phase shift of 360.

FIG. 7 illustrates a feedbackcontrol apparatus 112 that is generallysimilar to the previously described apparatus 12 of FIG. 1 but whichuses a three phase resolver instead of the quadrature resolver of theinitial embodiments, and in which the rotor and stator are reversed withregard to input and output functions. Thus, apparatus 112 includes threephase shift circuits 131, 132 and 133 with input circuit means forapplying the same signal E to all three of the circuits. Circuits 131and 132 produce output signals that are shifted in phase relative toeach other. Circuits 132 and 133 afford a phase shift of 120 withrespect to each other, the phase shift between the output signals ofcircuits 131 and 133 being 240.

The resolver 141 of feedback control apparatus 112 is again a rotarytransformer having three input windings 143, 144 and 145. Windings143-145 are disposed in 120 alignment with respect to each other; inthis instance the input windings are mounted on the rotor 148 of theresolver rather than on the stator. The three input windings'are allterminated at a common ground connection. Winding 143 is connected tothe output of phase shift circuit 131, winding 144 is connected to phaseshift circuit 132, and winding 145 is coupled to phase shift circuit133. Rotor 148 is connected to a suitable drive apparatus 147.

Again, the resolver is provided with an output winding 146 that performsthe same function as the winding 45 of resolver 41. Output winding 146,however, is a stator winding in the rotary transformer 141.

With the apparatus 112 illustrated in FIG. 7 the output signal E, canagain be demonstrated to conform to the expression given above inEquation 4; that is, the frequency shift of the feedback controlapparatus is again determined directly by the speed of rotation of therotor of the rotary resolver, the signal induced in output stage 146being a composite signal having a phase determined by the instantaneousangular orientation of this winding with respect to the input stagewindings 143-145.

In FIG. 7, the positions of the rotor and stator of the v resolver inthe signal channel coupling the microphone or other input device to theutilization device, such as the loudspeaker, are reversed as comparedwith the construction shown in FIG. 1. It is also possible to reversethe relative positions of the resolver and the phase-shift circuits inthe signal channel, as shown in FIG. 8.

Thus, FIG. 8 illustrates a feedback control apparatus 212 comprising aninductive rotary resolver 241 having a single input winding 246 and apair of output windings 242 and 243. Input winding 246 is a statorwinding and is coupled to a suitable signal source, not shown, thesignal E being applied thereto. Output windings 242 and 243 are mountedon a rotor in quadrature orientation to each other. One terminal ofwinding 242 is grounded and the other terminal is connected to a seriesRC phase shift circuit 232. One terminal of winding 243 is grounded andthe other terminal is connected to a parallel RC phase shift circuit233. The two phase shift circuits are connected together at a commonterminal 236.

As before, the rotor of resolver 241 is connected to a suitable drivemeans 247 that operates to rotate the rotor continuously duringoperation of the apparatus 212. In this form of the invention the inputsignal E is split into two components that are shifted in phase,relative to each other, by 90 plus a continuously varying factordetermined by the rotational speed of the resolver. These two componentsare recombined, by means of phase shift circuits 232 and 233, to producean output signal E having the same attributes, relative to the inputsignal E, as in the previous embodiments. Ofcourse, the apparatus 212would have a relatively narrow frequency range, as illustrated, but thiscan be broadened where necessary by replacing the simple phase shiftcircuits 232 and 233 with broad band circuits such as shown in FIG. 5.

By considering the basic functional attributes of each of the variousembodiments of the invention described above, their essential similarityis made readily apparent. I

In each form of the feedback control apparatus, the signal channelbetween the input device and the utilization device is split into two ormore sub-channels. The signals in the sub-channels are first subjectedto a substantially I. l. constant phase modification that is preferablya function of the number of sub-channels used; 90 is preferred with twochannels and 120 with three channels. Subsequently, the signals in thesub-channels are subjected to a complementary fixed phase modificationand the channels are recombined to again afford a single channel. Inaddition, a continuously varying phase shift is effected, with respectto the signals translated through the sub-channels, resulting in afrequency shift in the output signal from the apparatus.

In each embodiment, the rotary resolver performs two of the basicfunctions described above. Thus, the resolver simultaneously carries outthe varying phase shift operation and one of the two substantiallyconstant phase modification operations. The other fixed phasemodification is effected by conventional circuit means located eitherahead of or after the resolver in the signal channel.

Hence, while preferred embodiments of the invention have been describedand illustrated, it is to be understood that they are capable ofvariation and modification, and I therefore do not wish to be limited tothe precise details set forth, but desire to avail myself of suchchanges and alterations as fall within the purview of the followingclaims.

I claim:

1. Feedback control apparatus for minimizing oscillation tendencies in apublic address or like system including input means for developing aninitial signal varying over a wide frequency range, a signal channelcomprising amplifier means for amplifying that signal, and utilizationmeans for utilizing the amplified signal, and in which the utilizationmeans is coupled back to the input means through a feedback mediumproducing varying irregular gain at different frequencies within saidfrequency range, said feedback control apparatus comprising:

a rotary resolver, incorporated in said signal channel,

including three input stages disposed in 120 relation to each other andan output stage, coupled to all of said input stages, said resolveroutput stage being rotatable relative to said input stages progressivelyto vary the coupling of the output stage to the input stages in balancedthree-phase relation;

phase-shifting means, incorporated in said signal channel ahead of saidresolver and coupled to said input means, for developing and applying tosaid input stages three intermediate signals each corresponding to saidinitial signal but mutually differing in phase, relative to each other,by phase angles of 120 over at least a substantial portion of saidfrequency range;

drive means for rotating said output stage of said resolver at afrequency below said frequency range to produce, in the output stage ofsaid resolver, a utilization signal corresponding to said initial signalbut of continuously varying phase relative thereto;

and means for reversing the effective direction of rotation of saidresolver to change the direction of phase variation in said utilizationsignal.

2. Feedback control apparatus for minimizing oscillation tendencies in apublic address or like system including input means for developing aninitial signal varying over a wide frequency range, a signal channelcomprising amplifier means for amplifying that signal, and utilizationmeans for utilizing the amplified signal, and in which the utilizationmeans is coupled back to the input means through a feedback mediumproducing irregular gain at different frequencies within said frequencyrange, said feedback control apparatus comprising:

a rotary resolver capacitor incorporated in said signal channel andincluding a plurality of input stages each comprising a capacitor plate,and an output stage comprising a capacitor plate, further includingrotatable means for varying the capacitive coupling between saidcapacitor plate of said output stage and said capacitor plates of saidinput stages of said resolver;

phase-shifting means, incorporated in said signal channel ahead of saidresolver, for developing and applying to said resolver input stages acorresponding plurality of intermediate signals each corresponding tosaid initial signal but shifted in phase, relative to each other, by apredetermined amount;

drive means for continuously rotating said rotatable means in saidresolver at a constant frequency below said frequency range to produce,in the output stage of said resolver, a utilization signal correspondingto said initial signal but shifted in frequency by an amount directlyproportional to the rotational frequency of said rotary means of saidresolver;

and means for reversing the effective direction of rotation of saidresolver to change the sign of said frequency shifted.

3. Feedback control apparatus for minimizing oscillation tendencies in apublic address or like system including input means for developing aninitial signal varying over a wide frequency range, a signal channelcomprising amplifier means for amplifying that signal, and utilizationmeans for utilizing the amplified signal, and in which the utilizationmeans is coupled back to the input means through a feedback mediumproducing irregular gain at different frequencies within said frequencyrange, said feedback control apparatus comprising:

a rotary resolver incorporated in said signal channel and including aplurality of input stages, an output stage, and a rotor, said rotorbeing rotatable relative to said input stages to vary the coupling ofthe resolver output stage to each of said input stages in predeterminedratio;

phase-shifting means, incorporated in said signal channel ahead of saidresolver, for developing and applying to said input stages acorresponding plurality of intermediate signals each corresponding tosaid initial signal but shifted in phase, relative to each other, by apredetermined amount;

drive means for continuously rotating said resolver rotor at a constantfrequency below said frequency range to produce, in the output stage ofsaid resolver, a utilization signal corresponding to said initial signalbut of continuously varying phase relative thereto, the rate of phasechange being a direct function of the rotational speed of said rotor;

means for adjusting the operating speed of said drive means to modifythe rate of phase change to suit the environment of said system;

and means for reversing the elfective direction of rotation of saidresolver to change the direction of phase variation in said utilizationsignal.

4. Feedback control apparatus for minimizing oscillation tendencies in apublic address or like system including input means for developing aninitial signal varying over a wide frequency range, a signal channelcomprising amplifier means for amplifying that signal, and utilizationmeans for utilizing the amplified signal, and in which the utilizationmeans is coupled back to the input means through a feedback mediumproducing irregular gain at different frequencies Within said frequencyrange, said feedback control apparatus comprising:

a rotary resolver incorporated in said signal channel and including twoinput stages and an output stage, said resolver output stage beingrotatable relative to said input stages to vary the interstage couplingof the resolver;

phase-shifting means, incorporated in said signal channel ahead of saidresolver, for developing two intermediate signals each corresponding tosaid initial signal but shifted in phase, relative to each other, by apredetermined amount, and for applying each of said intermediate signalsto a respective one of said resolver input stages;

drive means for continuously rotating said resolver at a constantfrequency below said frequency range to produce, in the. output stage ofsaid resolver, a utilization signal corresponding to said initial signalbut of continuously varying phase relative thereto;

and switching means for reversing the connections of said phase-shiftingmeans to said resolver input stages to reverse the direction of phasechange effected by said resolver,

5. Feedback control apparatus for minimizing oscillation tendencies in apublic address or like system including input means for developing aninitial signal varying over a Wide frequency range, a signal channelcomprising amplifier means for amplifying that signal, and utilizationmeans for utilizing the amplified signal, and in which the utilizationmeans is coupled back to the input means through a feedback mediumproducing irregular gain at different frequencies Within said frequencyrange, said feedback control apparatus comprising:

a rotary resolver incorporated in said signal channel and including aninput stage and an output stage, one of said stages comprising aplurality of sub-stages, said resolver output and input stages beingrotatable relative to each other to vary the interstage coupling of theresolver;

phase-shifting means, comprising a corresponding plurality ofsub-channels individually incorporated in said signal channel in serieswith the sub-stages of said resolver, for shifting the signalstraversing said sub-channels in phase, relative to each other, by apredetermined amount;

drive means for continuously rotating said resolver at a constantfrequency substantially smaller than the principal portion of saidfrequency range to produce, in the output of said signal channel, autilization signal corresponding to said initial signal but shifted infrequency with respect thereto, the net frequency shift being directlyproportional to the rotational velocity of said resolver and of a signdetermined by the direction of rotation;

and means for reversing the effective direction of IO- tation of saidresolver to change the sign of said frequency shift.

References Cited by the Examiner UNITED STATES PATENTS 2,254,734 9/1941Falloon et al. 323-410 2,403,958 7/1946 Seeley 3231 13 2,553,558 5/1951Earp 32346 2,723,316 11/1955 Goodell ct a1. 1711 2,857,564 10/1958 Gray323113 3,105,877 10/1963 Miller et a1 179-1 FOREIGN PATENTS 626,912 7/1949 Great Britain.

ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner,

A. I. SANTORELLI, R. MURRAY, Assistant Examiners.

1. FEEDBACK CONTROL APPARATUS FOR MINIMIZING OSCILLATION TENDENCIES IN APUBLIC ADDRESS OR LIKE SYSTEM INCLUDING INPUT MEANS FOR DEVELOPING ANINITIAL SIGNAL VARYING OVER A WIDE FREQUENCY RANGE, A SIGNAL CHANNELCOMPRISING AMPLIFIER MEANS FOR AMPLIFYING THAT SIGNAL, AND UTILIZATIONMEANS FOR UTILIZING THE AMPLIFIED SIGNAL, AND IN WHICH THE UTILIZATIONMEANS IS COUPLED BACK TO THE INPUT MEANS THROUGH A FEEDBACK MEDIUMPRODUCING VARYING IRREGULAR GAIN AT DIFFERENT FREQUENCIES WITHIN SAIDFREQUENCY RANGE, SAID FEEDBACK CONTROL APPARATUS COMPRISING: A ROTARYRESOLVER, INCORPORATED IN SAID SIGNAL CHANNEL, INCLUDING THREE INPUTSTAGES DISPOSED IN 120* RELATION TO EACH OTHER AND AN OUTPUT STAGE,COUPLED TO ALL OF SAID INPUT STAGES, SAID RESOLVER OUTPUT STAGE BEINGROTATABLE RELATIVE TO SAID INPUT STAGES PROGRESSIVELY TO VARY THECOUPLING OF THE OUTPUT STAGE TO THE INPUT STAGES IN BALANCED THREE-PHASERELATION; PHASE-SHIFTING MEANS, INCORPORATED IN SAID SIGNAL CHANNELAHEAD OF SAID RESOLVER AND COUPLED TO SAID INPUT MEANS, FOR DEVELOPINGAND APPLYING TO SAID INPUT STAGES THREE INTERMEDIATE SIGNALS EACHCORRESPONDING TO SAID INITIAL SIGNAL BUT MUTUALLY DIFFERING IN PHASERELATIVE TO EACH OTHER, BY PHASE ANGLES OF 120* OVER AT LEAST ASUBSTANTIAL PORTION OF SAID FREQUENCY RANGE; DRIVE MEANS FOR ROTATINGSAID OUTPUT STAGE OF SAID RESOLVER AT A FREQUENCY BELOW SAID FREQUENCYRANGE TO PRODUCE, IN THE OUTPUT STAGE OF SAID RESOLVER, A UTILIZATIONSIGNAL CORRESPONDING TO SAID INITIAL SIGNAL BUT OF CONTINUOUSLY VARYINGPHASE RELATIVE THERETO; AND MEANS FOR REVERSING THE EFFECTIVE DIRECTIONOF ROTATION OF SAID RESOLVER TO CHANGE THE DIRECTION OF PHASE VARIATIONIN SAID UTILIZATION SIGNAL.